Sheffield United's Under 23s failed to build on last week's opening day win at Cardiff City with defeat in their first home match at Chesterfield's Proact Stadium against Watford.
In a close fought game, Watford took the lead in the second half before a wave of Blades pressure resulted in Ompreon's equaliser, but the killer blow came in the 93rd minute when a Watford trialist tapped home at the back post.
The opening 20 minutes of the match thought brought few chances as both sides eased their way into the game, but it was United who went closest when Jake Young saw a low effort pushed wide by the goalkeeper.
Jake Wright made a big block down the other end soon after to deny Watford a goal scoring opportunity before Young went even closer, seeing a close-range effort cleared off the line after good work down the left from Ompreon.
Like at Cardiff last week, most of the action was saved for the second half, but it was the visitors who took the lead when Hinds forced Dewhurst into spilling a loose ball and Hungbo was on hand to roll the ball into an empty net.
The goal sparked some life into the Blades who quickly went on the offensive in search of an equaliser. Harry Boyes saw a shot blocked before Keenan Ferguson headed over the bar at the back post.
It felt like a goal was coming when Young saw a header pushed just wide, and it did come a minute later after some quick passing released Ompreon who fired low past the goalkeeper to equalise.
The momentum had shifted United's way but it was Watford who went closest as Wright had to be in the right place to clear a header off the line.
Kyron Gordon saw a header of his own saved at the other end, but the final blow came with the last attack of the game when a low cross reached the trialist at the back post who promptly fired home from close range to win the game for Watford.
Blades: Dewhurst, Ferguson, Boyes, Gordon, Wright, Belehouan, Broadbent, York, Young, Brunt (Ndiaye 71), Ompreon.
Unused: Sheppeard, Amissah, Hall, Trialist.